An influential conventional wisdom holds that civil wars proliferated rapidly with the end of the Cold War and that the root cause of many or most of these has been ethnic and religious antagonisms. We show that the current prevalence of internal war is mainly the result of a steady accumulation of protracted conflicts since the 1950s and 1960s rather than a sudden change associated with a new, post-Cold War international system. We also find that after controlling for per capita income, more ethnically or religiously diverse countries have been no more likely to experience significant civil violence in this period. We argue for understanding civil war in this period in terms of insurgency or rural guerrilla warfare, a particular form of military practice that can be harnessed to diverse political agendas. The factors that explain which countries have been at risk for civil war are not their ethnic or religious characteristics but rather the conditions that favor insurgency. These include poverty—which marks financially and bureaucratically weak states and also favors rebel recruitment—political instability, rough terrain, and large populations.We wish to thank the many people who provided comments on earlier versions of this paper in a series of seminar presentations. The authors also gratefully acknowledge the support of the National Science Foundation (Grants SES-9876477 and SES-9876530); support from the Center for Advanced Study in the Behavioral Sciences with funds from the William and Flora Hewlett Foundation; valuable research assistance from Ebru Erdem, Nikolay Marinov, Quinn Mecham, David Patel, and TQ Shang; sharing of data by Paul Collier.